How long will your battery run the load? Works for 12V, lithium/LiFePO4, AGM and UPS batteries — DC or AC through an inverter.
Used to convert amp-hours to energy. Also converts an amp load to watts.
Typical pure-sine inverters run 85–92%. Lower = more wasted as heat.
How much of the time the load actually draws power. Constant loads = 100%. A cycling fridge/compressor is often 30–50%.
An estimate. Real runtime is also affected by battery age, temperature (cold cuts capacity), and — for lead-acid — the Peukert effect at high discharge rates. Build in a safety margin and never plan to fully drain a battery.
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How battery runtime is calculated
The core idea is simple: runtime is the usable energy in the battery divided by how fast the load drains it.
Runtime (h) = (Capacity in Wh × DoD) ÷ Load in W
Three things turn that clean formula into a realistic answer, and this calculator handles all of them:
1. Usable capacity, not rated capacity
You can't drain a battery to zero without damaging it. The fraction you can safely use is the depth of discharge (DoD). Lead-acid and AGM batteries are typically held to 50% DoD; LiFePO4 lithium can go to 80% routinely and 100% occasionally. So a "100 Ah" lead-acid battery realistically gives you 50 Ah of work, while a 100 Ah LiFePO4 gives you 80 Ah.
2. Inverter losses on AC loads
If your appliance plugs into a wall socket, the power passes through an inverter (or a UPS) that converts the battery's DC into AC — and that conversion wastes 8–15% as heat. The battery therefore has to supply more than the appliance's rated watts. This calculator divides the load by the inverter efficiency when you select an AC load. DC loads wired straight to the battery skip this step.
3. Duty cycle for things that cycle
A fridge is the classic trap. Its compressor doesn't run constantly — it switches on to pull the temperature down, then coasts. Over a full day a 60 W fridge might only draw power 40% of the time, averaging 24 W. If you assume it runs flat-out you'll badly under-estimate runtime. Set the duty cycle to match how often the load is actually on.
A worked example
A 12 V 100 Ah LiFePO4 battery running a 100 W TV through a 90% inverter, constant:
Energy: 100 Ah × 12 V = 1,200 Wh
Usable at 80% DoD: 960 Wh
Draw from battery: 100 W ÷ 0.90 = 111 W
Runtime: 960 ÷ 111 ≈ 8.6 hours
Swap the TV for a 12 V DC fridge averaging 45 W at a 40% duty cycle (18 W average, no inverter) and the same battery lasts over two days — a vivid illustration of why DC appliances and duty cycle matter so much in van and off-grid builds.
Typical loads for reference
Appliance
Running watts
Notes
LED light
5–15 W
Constant
Laptop charger
45–90 W
Tapers as it charges
12 V DC compressor fridge
40–60 W
~30–50% duty cycle
Starlink dish
50–75 W
Mostly constant
Microwave (small)
700–1000 W
Short bursts
Electric kettle
1200–1500 W
Short bursts, no surge
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Frequently asked questions
How do you calculate battery runtime?
Divide the usable energy by the load: runtime (hours) = (capacity in Wh × DoD) ÷ load in watts. Convert an Ah rating to Wh by multiplying by the battery voltage. If the load runs through an inverter, divide the load by the inverter's efficiency first.
How long will a 100Ah 12V battery last?
A 100Ah 12V battery holds 1,200 Wh. Usable energy is about 960 Wh for LiFePO4 (80% DoD) or 600 Wh for lead-acid (50% DoD). Divide by your load: a 100 W appliance lasts roughly 9.6 hours on LiFePO4 or 6 hours on lead-acid, before inverter losses.
Why does my battery run for less time than the math says?
Three reasons: depth of discharge (don't drain to zero), inverter losses (8–15% wasted converting DC to AC), and the Peukert effect (lead-acid delivers less at high discharge rates). Cold also reduces capacity. This calculator handles DoD and inverter efficiency; add a margin for the rest.
What is duty cycle and why does it matter for a fridge?
A fridge compressor cycles on and off to hold temperature — typically running 30–50% of the time. A fridge "rated" 60 W might average only 25 W. Setting a duty cycle below 100% reflects this and gives a far more realistic runtime than assuming nonstop running.
Does this work for a UPS or inverter battery?
Yes. For a UPS or inverter backup, set the load type to AC so conversion losses are included. Enter the battery capacity (UPS units often use a 12V 7Ah or 9Ah block, sometimes several) and your equipment's real power draw in watts.